The key difference between hemocyanin and hemoglobin is that hemocyanin is a copper-containing extracellular respiratory pigment present in some invertebrate blood while haemoglobin is an iron-containing intracellular respiratory pigment present in vertebrate blood.
In aerobic organisms, gas exchange occurs through the metalloproteins present in the blood. Therefore, hemocyanin and hemoglobin are two metalloproteins that facilitate gas exchange in invertebrate animals and vertebrate animals, respectively. Hemocyanin is a copper-containing respiratory pigment which is found suspended in the hemolymph of invertebrates. In contrast, hemoglobin is an iron-containing respiratory pigment that is bound to the red blood cells of vertebrates. The oxygenated form of hemocyanin is blue in colour. But, the oxygenated form of hemoglobin is bright red in colour.
CONTENTS
1. Overview and Key Difference
2. What is Hemocyanin
3. What is Hemoglobin
4. Similarities Between Hemocyanin and Hemoglobin
5. Side by Side Comparison – Hemocyanin vs Hemoglobin in Tabular Form
6. Summary
What is Hemocyanin?
Hemocyanin is a respiratory pigment present in some invertebrate animals, especially in mollusks. It is a copper-containing metalloprotein that shows an affinity to oxygen. Therefore, it carries out a similar function as the hemoglobin in vertebrates. But, unlike hemoglobin, hemocyanin is not bound to any cell. Instead, it is suspended directly in the hemolymph and transport oxygen through the body. Therefore, they are free-floating proteins in the blood. Originally, hemocyanin is colourless. Once it is bound with oxygen, it becomes blue in colour.
Figure 01: Hemocyanin
Structurally, hemocyanin is composed of many subunits containing imidazole rings of six histidine residues. Each subunit weighs about 75 kilodaltons (kDa). Since there are many subunits, hemocyanin is a large molecule which has a high molecular weight compared to hemoglobin. Furthermore, according to investigations, it has been found that hemocyanin is species-specific. Arthropods and mollusks have different types of hemocyanins.
What is Hemoglobin?
Hemoglobin (Hgb) is a vital metalloprotein molecule present in vertebrate red blood cells which transports oxygen from the lungs to other body tissues and carbon dioxide from the body tissues to the lungs. Thus, it works as a respiratory pigment. Hemoglobin molecule is made up of four sub protein molecules in which two chains are alpha globulin chains and the other two are beta globulin chains. In each globulin chain, there is an iron-containing porphyrin compound called heme group. Within each heme group, there is an embedded iron atom. These iron-containing hemoglobin proteins are responsible for the red colour of the blood. Structurally, haemoglobin is composed of C, H, N, and O.
Figure 02: Hemoglobin
Hemoglobin is the main protein molecule which provides the typical shape of the red blood cell, that is the round shape with a narrow center. Iron atoms and the shape of the red blood cells are crucial for transporting oxygen through the blood. If the shape of hemoglobin is destroyed, it fails to transport oxygen. Sickle cell hemoglobin is one type of abnormal hemoglobin molecule which causes the anemia conditions called sickle cell anemia.
In normal hemoglobin, in beta chains, 6th position of the amino acid chain is composed of glutamic acid. However, in sickle cell hemoglobins, 6th position is taken up by a different amino acid called valine. Though it is a single amino acid difference, it is responsible for this life-threatening anemia condition.
Generally, hemoglobin shows a higher affinity for oxygen since there are four oxygen binding sites located inside a hemoglobin molecule. Once hemoglobin molecule is saturated with oxygen, the blood becomes bright red in colour and this state is known as oxygenated blood. The second state of the hemoglobin is known as deoxyhemoglobin which lacks oxygen. At this state, blood bears the dark red colour.
What are the Similarities Between Hemocyanin and Hemoglobin?
- Hemocyanin and hemoglobin are proteins.
- Both types of molecules work as respiratory pigments.
- They carry oxygen to body tissues.
- Therefore, they can bind with oxygen.
What is the Difference Between Hemocyanin and Hemoglobin?
Hemocyanin is a copper-containing protein present suspended in the hemolymph of invertebrates that transports oxygen within the body. On the other hand, hemoglobin is an iron-containing protein in the red blood cells of vertebrates which transports oxygen and carbon dioxide through the blood. So, this is the key difference between hemocyanin and hemoglobin. Structurally, hemocyanin is composed of many protein subunits, while hemoglobin is composed of two alpha chains and two beta chains.
Moreover, hemocyanin is a free-floating protein, while hemoglobin is bound to red blood cells. Another important difference between hemocyanin and hemoglobin is that the central ion of hemocyanin is copper while the central ion of hemoglobin is iron. Most importantly, the colour of hemocyanin is blue while the colour of hemoglobin is red. Therefore, this another major difference between hemocyanin and hemoglobin.
Summary – Hemocyanin vs Hemoglobin
Hemocyanin and hemoglobin are two metalloproteins that transport oxygen through the blood. Therefore, they are respiratory pigments that work as oxygen carriers. The key difference between hemocyanin and hemoglobin is that hemocyanin is a copper-containing extracellular protein while hemoglobin is an iron-containing intracellular protein. Moreover, hemocyanins are found in invertebrate animals, especially in mollusks and arthropods, while hemoglobins are found in vertebrate blood. Furthermore, the colour of oxygenated hemocyanin is blue while the colour of the oxygenated hemoglobin is red.
Reference:
1. Holde, Kensal E. van, and Karen I. Miller. “Hemocyanins and Invertebrate Evolution.” Journal of Biological Chemistry, 11 May 2001, Available here.
2. “Hemoglobin.” Wikipedia, Wikimedia Foundation, 19 Dec. 2019, Available here.
Image Courtesy:
1. “Hemocyanin2” By Cuff ME, Miller KI, van Holde KE, Hendrickson WA (Public Domain) via Commons Wikimedia
2. “1904 Hemoglobin” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia